Investigating the Electrochemical Performance of MnFe2O4@xC Nanocomposites as Anode Materials for Sodium-Ion Batteries

Transition metal oxides (TMOs) are important anode materials in sodium-ion batteries (SIBs) due to their high theoretical capacities, abundant resources, and cost-effectiveness. However, issues such as the low conductivity and large volume variation of TMO bulk materials during the cycling process result in poor electrochemical performance. Nanosizing and compositing with carbon materials are two effective strategies to overcome these issues. In this study, spherical MnFe2O4@xC nanocomposites composed of MnFe2O4 inner cores and tunable carbon shell thicknesses were successfully prepared and utilized as anode materials for SIBs. It was found that the property of the carbon shell plays a crucial role in tuning the electrochemical performance of MnFe2O4@xC nanocomposites and an appropriate carbon shell thickness (content) leads to the optimal battery performance. Thus, compared to MnFe2O4@1C and MnFe2O4@8C, MnFe2O4@4C nanocomposite exhibits optimal electrochemical performance by releasing a reversible specific capacity of around 308 mAh·g−1 at 0.1 A·g−1 with 93% capacity retention after 100 cycles, 250 mAh·g−1 at 1.0 A g−1 with 73% capacity retention after 300 cycles in a half cell, and around 111 mAh·g−1 at 1.0 C when coupled with a Na3V2(PO4)3 (NVP) cathode in a full SIB cell.


Introduction
With the growing depletion of traditional fossil fuels and the resultant environmental challenges, new energy sources with robust advancement have emerged as a pivotal concern for the sustainable progress of human society.Currently, intermittent renewable energy sources such as solar energy, hydropower, and wind energy are affected by natural conditions, such as weather changes and climate anomalies, which affect the energy supply and cannot guarantee stability [1,2].Consequently, the development of energy storage technology has become increasingly vital in addressing the stability issues associated with energy supply [3].Among numerous secondary energy systems, sodium-ion batteries (SIBs) have been widely studied as an alternative to lithium-ion batteries (LIBs) due to their abundant and homogeneous sodium resources, low cost, and high safety ratings [4].To date, a plethora of conversion-based materials, such as transition metal oxides (TMOs)/sulfides/phosphides/selenides [5][6][7][8], carbon-based materials [9], alloying materials [10], and organic compounds [11], have demonstrated fantastic electrochemical activity as anode materials for SIBs.
Among all these anode materials, TMOs have piqued the interest of researchers because of their abundant redox-active properties, low cost, high energy density, ease of preparation, and environmental benignity [12].TMOs such as MnFe 2 O 4 [13], CuMn 2 O 4 [14], FeCo 2 O 4 [15], FeV 2 O 4 [16], ZnFe 2 O 4 [17], ZnCo 2 O 4 [18], and ZnMn 2 O 4 [19] have been explored as anode materials for SIBs.However, the intrinsically poor electric conductivity and uncontrollable and inevitable volume changes in bulk TMO electrode materials during the cycling process lead to poor electrochemical performance [20,21], seriously hindering the practical application of TMO-based electrode materials.Composite formation and nanosizing of carbon materials are two effective strategies to solve the above-mentioned problems.Nanosizing of the carbon composite can improve the effective contact area of the TMO electrode with electrolytes, mitigating irreversible damage to the composition of the electrode caused by volume changes during the charge-discharge cycle, shortening ion diffusion distance and, thus, enhancing the specific capacity, rate capability, and long-term cycle reversibility of batteries.Composite formation with carbon materials [17][18][19][21][22][23][24], especially the coating of a carbon shell onto TMO nanoparticles, enhances the electric conductivity, prevents the agglomeration of nanoparticles, and strengthens the mechanical integrity of electrode materials, thereby improving the overall electrochemical performance of batteries.
In recent years, nanocomposite materials composed of TMO cores and carbon shells have been widely studied to improve the conductivity and mechanical properties of TMOs@C to improve their electrochemical performance as anode materials for SIBs [17,23,24].Studies of alloying-conversion-type ZnFe 2 O 4 @xC SIB anode materials have shown that enhanced electrochemical performance can be achieved by increasing the carbon shell thickness of the nanocomposites [17].In this work, we synthesized conversion-type MnFe 2 O 4 spherical nanoparticles using a hydrothermal method and coated them with different thicknesses of carbon shell by tuning the mass of polyvinylidene fluoride (PVDF) for pyrolysis to prepare core-shell-structure MnFe 2 O 4 @xC nanocomposites to study the influence of the thickness of the carbon shell on the electrochemical performance of conversiontype SIB anode materials.MnFe 2 O 4 @xC nanocomposites with carbon shell contents of 3.28%, 12.89%, and 26.30% (mass ratio) are denoted as MnFe 2 O 4 @1C, MnFe 2 O 4 @4C, and MnFe 2 O 4 @8C, respectively.The sodium-ion storage performance of these nanocomposites was explored in a half cell.At a specific current of 0.1 A•g −1 , MnFe 2 O 4 @4C (308 mAh•g −1 ) and MnFe 2 O 4 @1C (295 mAh•g −1 ) nanocomposites show higher specific capacities than MnFe 2 O 4 @8C (195 mAh•g −1 ), while MnFe 2 O 4 @4C and MnFe 2 O 4 @8C show better cycling stabilities than MnFe 2 O 4 @1C in a comparison of 93%, 99%, and 71% capacity retentions after 100 cycles.A similar phenomenon can be observed when the specific current is 1.0 A g −1 .MnFe 2 O 4 @4C and MnFe 2 O 4 @8C show better rate capabilities than MnFe 2 O 4 @1C.With 40 folds of specific current increment (from 0.05 to 2.0 A g −1 ), the capacity retentions of MnFe 2 O 4 @4C, MnFe 2 O 4 @8C, and MnFe 2 O 4 @1C are 65%, 63%, and 44%, respectively.
Transmission electron microscopy (TEM), electrochemical impedance spectroscopy (EIS), and kinetic analysis show that the carbon shell plays a crucial role in tuning the electrochemical performance of MnFe 2 O 4 @xC nanocomposites.The electric conductivity and the connectivity among particles of MnFe 2 O 4 @xC nanocomposites increase with the thickness of the carbon shell (content), which leads to enhanced rate capability and cycling stability.Therefore, MnFe 2 O 4 @4C and MnFe 2 O 4 @8C show superior rate capability and cycling stability relative to MnFe 2 O 4 @1C.Nevertheless, further increasing the thickness of the carbon shell results in a smaller ratio of active MnFe 2 O 4 core material (the specific capacity was calculated based on the mass of the whole MnFe 2 O 4 @xC nanocomposites), leading to a decreased specific capacity, which explains the lower specific capacity of MnFe 2 O 4 @8C than that of MnFe 2 O 4 @1C and MnFe 2 O 4 @4C.In summary, MnFe 2 O 4 @4C with an appropriate carbon shell thickness (content) leads to optimal battery performance with a reversible specific capacity of around 308 mAh•g −1 at 0.1 A•g −1 with 93% capacity retention after 100 cycles and 250 mAh•g −1 at 1.0 A g −1 with 73% capacity retention after 300 cycles in a half cell.Finally, the full SIB cell performance of a MnFe 2 O 4 @4C anode coupled with a Na 3 V 2 (PO 4 ) 3 (NVP) cathode was also tested, showing a specific capacity of around 111 mAh•g −1 at 1.0 C. Our findings show the importance of tuning the thickness of the carbon shell to achieve superior battery performance of TMO SIB anodes.
with a Na3V2(PO4)3 (NVP) cathode was also tested, showing a specific capacity of around 111 mAh•g −1 at 1.0 C. Our findings show the importance of tuning the thickness of the carbon shell to achieve superior battery performance of TMO SIB anodes.

Structure and Morphology Analysis
The crystallinity and phase purity of the MnFe2O4, MnFe2O4@1C, MnFe2O4@4C, and MnFe2O4@8C samples were characterized using an X-ray diffraction (XRD) technique, the results of which are shown in Figures 1a and S2b, indicating the cubic property with the Fd-3m space group of both materials [25,26].The main peaks at 2θ (in degree) values of 18.1°, 29.7°, 35.0°, 42.5°, 56.2°, and 61.7° of the XRD spectra correspond to (1 1 1), (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 4 0) crystal planes, respectively [27].They are essentially consistent with the MnFe2O4 standard database (PDF#10-0319).In addition, the peak positions of the diffraction peaks of the MnFe2O4 and MnFe2O4@1C samples are similar to each other, indicating that the carbon coating did not change the crystallinity and phase of MnFe2O4.The Raman spectra of the MnFe2O4@1C, MnFe2O4@4C, and MnFe2O4@8C nanocomposites (Figure 1b) show two prominent peaks at approximately 1330 cm −1 and 1590 cm −1 , which correspond to the D and G bands, respectively, indicating the complete calcination of PVDF into carbon.Figure 1c shows a SEM image of MnFe2O4@1C nanocomposite, and the results reveal that the synthesized material primarily consists of spherical nanoparticles with a diameter of approximately 400 nm.The SEM images of MnFe2O4 also show similar spherical nanoparticles (Figure S1).The TEM images of MnFe2O4@1C in Figure 1d,e show that the MnFe2O4 inner core is surrounded by a thin carbon coating layer of approximately 10 nm.The element mapping images of the MnFe2O4@1C nanocomposite are depicted in Figure 1f-i.The evident distribution of Mn, Fe, O, and C elements in the MnFe2O4@1C nanocomposites further indicates the spherical core-shell nanostructure of the MnFe2O4@1C nanocomposite, with the C element shell loaded on the surface of the MnFe2O4 inner core.The HRTEM image of MnFe2O4@4C in Figure 2a illustrates that the MnFe2O4 core is Figure 1c shows a SEM image of MnFe 2 O 4 @1C nanocomposite, and the results reveal that the synthesized material primarily consists of spherical nanoparticles with a diameter of approximately 400 nm.The SEM images of MnFe 2 O 4 also show similar spherical nanoparticles (Figure S1).The TEM images of MnFe 2 O 4 @1C in Figure 1d,e show that the MnFe 2 O 4 inner core is surrounded by a thin carbon coating layer of approximately 10 nm.The element mapping images of the MnFe 2 O 4 @1C nanocomposite are depicted in Figure 1f-i.The evident distribution of Mn, Fe, O, and C elements in the MnFe 2 O 4 @1C nanocomposites further indicates the spherical core-shell nanostructure of the MnFe 2 O 4 @1C nanocomposite, with the C element shell loaded on the surface of the MnFe 2 O 4 inner core.The HRTEM image of MnFe 2 O 4 @4C in Figure 2a illustrates that the MnFe 2 O 4 core is surrounded by a carbon coating of 20 to 30 nm.Furthermore, crystal plane stripes with lattice spacings of 0.25 and 0.30 nm are depicted in Figure 2b, which can be attributed to the (3 1 1) and (2 2 0) crystal planes of MnFe 2 O 4 core, respectively [28].TEM images of the MnFe 2 O 4 @8C nanocomposite in Figure 2c,d show that the thickness of the carbon shell is approximately 100 nm.As can be observed from the TEM images, the thickness of the carbon shell increases following a sequence from MnFe 2 O 4 @1C and MnFe 2 O 4 @4C to MnFe 2 O 4 @8C, which enhances the connectivity among nanoparticles.
surrounded by a carbon coating of 20 to 30 nm.Furthermore, crystal plane stripes with lattice spacings of 0.25 and 0.30 nm are depicted in Figure 2b, which can be attributed to the (3 1 1) and (2 2 0) crystal planes of MnFe2O4 core, respectively [28].TEM images of the MnFe2O4@8C nanocomposite in Figure 2c,d show that the thickness of the carbon shell is approximately 100 nm.As can be observed from the TEM images, the thickness of the carbon shell increases following a sequence from MnFe2O4@1C and MnFe2O4@4C to MnFe2O4@8C, which enhances the connectivity among nanoparticles.The thermal decomposition process of MnFe2O4@xC was measured in an Ar atmosphere and is shown in Figure S2b.The three MnFe2O4@xC samples exhibit similar TGA curves, with weight loss primarily occurring at >450 °C, which can be attributed to the loss of carbon coating in the outer layer of the composite materials.The residual mass at the end of the TG curve is attributed to the residue of MnFe2O4, demonstrating the thermal stability of MnFe2O4 core material, even at 1000 °C.Therefore, the carbon content in the three nanocomposite materials is 3.28%, 12.89%, and 26.30%, as denoted by MnFe2O4@1C, MnFe2O4@4C, and MnFe2O4@8C, respectively.

Sodium-Ion Storage Performance of the MnFe2O4@xC Nanocomposites
The Na + storage performance of the three MnFe2O4@xC nanocomposites was evaluated, as presented in Figures 3 and 4. As shown in Figure 3a-c, the CV curves of the three materials show similar features.During the first cathodic scan, a sharp cathodic peak at ~0.17 V is observed before disappearing in the subsequent cycle, which is mainly attributed to irreversible electrochemical processes such as the transformation of MnFe2O4 to metallic Mn and Fe [29] and the formation of SEI, as represented in Equation (1).The anodic peak around 1.34 V is assigned to the oxidation of Mn and Fe, leading to the formation of MnO and Fe2O3, respectively, as indicated in Equation (2).The thermal decomposition process of MnFe 2 O 4 @xC was measured in an Ar atmosphere and is shown in Figure S2b.The three MnFe 2 O 4 @xC samples exhibit similar TGA curves, with weight loss primarily occurring at >450 • C, which can be attributed to the loss of carbon coating in the outer layer of the composite materials.The residual mass at the end of the TG curve is attributed to the residue of MnFe 2 O 4 , demonstrating the thermal stability of MnFe 2 O 4 core material, even at 1000 • C. Therefore, the carbon content in the three nanocomposite materials is 3.28%, 12.89%, and 26.30%, as denoted by MnFe 2 O 4 @1C, MnFe 2 O 4 @4C, and MnFe 2 O 4 @8C, respectively.

Sodium-Ion Storage Performance of the MnFe 2 O 4 @xC Nanocomposites
The Na + storage performance of the three MnFe 2 O 4 @xC nanocomposites was evaluated, as presented in Figures 3 and 4. As shown in Figure 3a-c, the CV curves of the three materials show similar features.During the first cathodic scan, a sharp cathodic peak at ~0.17 V is observed before disappearing in the subsequent cycle, which is mainly attributed to irreversible electrochemical processes such as the transformation of MnFe 2 O 4 to metallic Mn and Fe [29] and the formation of SEI, as represented in Equation (1).The anodic peak around 1.34 V is assigned to the oxidation of Mn and Fe, leading to the formation of MnO and Fe 2 O 3 , respectively, as indicated in Equation (2).
In the second and third cycles, a cathodic peak shows up at ~0.76 V, indicating the reversible reduction reactions of MnO to Mn and Fe 2 O 3 to Fe (Equations ( 3) and ( 4)) [30,31].From the second cycle onwards, it is noted that the majority of the CV curves overlap, indicating the favorable reversibility of the electrochemical reactions, as described in Equations ( 5) and ( 6) [13,[32][33][34].
term cycling performance of MnFe2O4@1C, MnFe2O4@4C, and MnFe2O4@8C electrodes at 0.1 A•g −1 .The results indicate that after 100 cycles, the three electrodes achieved discharge capacities of 220, 274, and 193 mAh•g −1 from the initial discharge capacities of 308, 295, and 195 mAh•g −1 , with capacity retentions of 71%, 93%, and 99%, respectively.At a specific current of 1.0 A•g −1 (Figure 4c), the initial capacities of the MnFe2O4@4C and MnFe2O4@8C electrodes are 250 and 166 mAh•g −1 , decaying to 183 and 129 mAh•g −1 after 300 cycles, indicating capacity retentions of around 73% and 78%, respectively.The MnFe2O4@1C electrode shows inferior cycling stability relative to the other two electrodes.In summary, MnFe2O4@1C and MnFe2O4@4C show higher specific capacity than MnFe2O4@8C, while MnFe2O4@4C and MnFe2O4@8C show superior rate capability and cycling stability relative to MnFe2O4@1C.Comprehensively considering the specific capacity, rate capability, and cycling stability, MnFe2O4@4C achieves optimal electrochemical battery performance.A comparison of the SIB anode performance of the MnFe2O4@4C nanocomposite with other TMOs is shown in Table 1, revealing the superior electrochemical performance of our material.Ref.
CuMn2O4/ graphene 0.01-3.0313/50th/0.1 145/2.0[14] Fe2O3-600 nanosheets 0.01-3.0~100/250th/0.5100/0.5 [35] 5b).The cell resistances (R s , calculated from the intercept of the X-axis) for the MnFe 2 O 4 @1C, MnFe 2 O 4 @4C, and MnFe 2 O 4 @8C Na half cells are 1.86, 1.59, and 1.54 Ω, respectively.The diameters of the semicircles represent the charge transfer resistance (R ct ) coming from the electrode material/electrolyte interface reaction, which are 0.84, 0.70, and 0.62 Ω for the MnFe 2 O 4 @1C, MnFe 2 O 4 @4C, and MnFe 2 O 4 @8C nanocomposite electrodes, respectively.The R ct increases gradually with the increase in the thickness (content) of the carbon shell in the core-shell nanostructure due to the good electric conductivity of the carbon material, which improves the electric conductivity of the MnFe 2 O 4 @xC core-shell nanocomposites as a whole.Good electric conductivity leads to good rate capability.Moreover, as observed in Figures 1 and 2, the particle connectivity of MnFe 2 O 4 @xC nanocompos-ites increases with the increase in carbon shell thickness (content), which leads to enhanced rate capability and cycling stability.Therefore, MnFe 2 O 4 @4C and MnFe 2 O 4 @8C show superior rate capability and cycling stability relative to MnFe 2 O 4 @1C.Nevertheless, a thick carbon shell results in a small ratio of active MnFe 2 O 4 core material (the specific capacity was calculated based on the mass of the whole MnFe 2 O 4 @4C nanocomposite), leading to a decreased specific capacity, which explains the lower specific capacity of MnFe 2 O 4 @8C relative to MnFe 2 O 4 @1C and MnFe 2 O 4 @4C.In summary, the use of MnFe 2 O 4 @4C with an appropriate carbon shell thickness (content) leads to optimal battery performance.Figure 5a illustrates the results of an EIS test on the three MnFe2O4@xC nanocom sites.The Nyquist curves of the three materials exhibit similar trends, comprising a s circle in the high-frequency region and an inclined straight line in the low-frequenc gion (Figure 5b).The cell resistances (Rs, calculated from the intercept of the X-axis the MnFe2O4@1C, MnFe2O4@4C, and MnFe2O4@8C Na half cells are 1.86, 1.59, and 1.5 respectively.The diameters of the semicircles represent the charge transfer resistance coming from the electrode material/electrolyte interface reaction, which are 0.84, 0.70 0.62 Ω for the MnFe2O4@1C, MnFe2O4@4C, and MnFe2O4@8C nanocomposite electro respectively.The Rct increases gradually with the increase in the thickness (content) o carbon shell in the core-shell nanostructure due to the good electric conductivity o carbon material, which improves the electric conductivity of the MnFe2O4@xC corenanocomposites as a whole.Good electric conductivity leads to good rate capab Moreover, as observed in Figures 1 and 2, the particle connectivity of MnFe2O4@xC n composites increases with the increase in carbon shell thickness (content), which lea enhanced rate capability and cycling stability.Therefore, MnFe2O4@4C and MnFe2O show superior rate capability and cycling stability relative to MnFe2O4@1C.Neverthe a thick carbon shell results in a small ratio of active MnFe2O4 core material (the spe capacity was calculated based on the mass of the whole MnFe2O4@4C nanocompo leading to a decreased specific capacity, which explains the lower specific capaci MnFe2O4@8C relative to MnFe2O4@1C and MnFe2O4@4C.In summary, the us MnFe2O4@4C with an appropriate carbon shell thickness (content) leads to optimal ba performance.To systematically investigate the electrochemical reaction kinetics of MnFe 2 O 4 @4C as an anodic electrode material for Na half cells, the corresponding CV curves were tested, with the scan rate increasing from 0.2 to 1.6 mV•s −1 (Figure 6a).The CV curves of MnFe 2 O 4 @4C at different scanning rates exhibit similar shapes, indicating that the material has low polarization and good electrochemical reaction kinetics.Under different scanning rates (v), the capacitance behavior or diffusion behavior of the electrode is obtained by deducing the current (i) according to the following formula (Equations ( 7) and ( 8)) [43][44][45] where a is a constant and b represents the capacitance ratio, which can be determined by the slope of the logarithmic curve (log (i) vs. log (v)).A value of b = 0.5 indicates a diffusion-controlled behavior, while a value of b = 1.0 signifies a pseudocapacitancecontrolled contribution.In Figure 6b, the slope of the redox process is depicted, with the b values of cathodic and anodic currents being 0.86 and 0.88, respectively.This suggests that the sodium storage behavior in MnFe 2 O 4 @4C is primarily influenced by pseudocapacitance.Additionally, we consider Equation (9) [46], where the current (i) at a fixed potential (V) in CV can be divided into two parts, namely capacitance behavior (k 1 v) and diffusion control (k 2 v 1/2 ).The capacitive contribution of the MnFe 2 O 4 @4C half cell at 1.0 mV•s −1 is shown in Figure 6c, which is measured to be 95.6% of the total contribution.The respective contributions of capacitance behavior and diffusion control at different scan rates are depicted in Figure 6d.The contribution of capacitance behavior increases as the scan rate rises, indicating that the pseudocapacitance process in the MnFe 2 O 4 @4C Na half cell becomes more prominent at higher scan rates, demonstrating faster kinetics.At a scanning rate of 1.6 mV•s −1 , a capacitance contribution of 97.5% is achieved.Even at a low rate of 0.2 mV•s −1 , MnFe 2 O 4 @4C still significantly contributes to the capacitance behavior (92.2%).MnFe 2 O 4 @1C and MnFe 2 O 4 @8C show similar electrochemical reaction kinetics to those of MnFe 2 O 4 @4C, with the b values of cathodic and anodic currents of the MnFe 2 O 4 @1C electrode of 0.85 and 0.84, respectively, while those of the MnFe 2 O 4 @8C electrode are 0.86 and 0.90, respectively (Figure S3).

(d)
The ratio of capacitive contribution of the MnFe2O4@4C anode at various scan rates.

Full-Cell Performance
To further assess the practical usability of MnFe2O4@4C in SIBs, full-cell configurations were assembled, with Na3V2(PO4)3 (NVP) serving as the cathode and MnFe2O4@4C as the anode (Figure 7a).The charge-discharge profiles of the NVP//MnFe2O4@4C full SIB

Full-Cell Performance
To further assess the practical usability of MnFe 2 O 4 @4C in SIBs, full-cell configurations were assembled, with Na 3 V 2 (PO 4 ) 3 (NVP) serving as the cathode and MnFe 2 O 4 @4C as the anode (Figure 7a).The charge-discharge profiles of the NVP//MnFe 2 O 4 @4C full SIB at 1 C-rate (1 C-rate = 120 mA•g −1 ) within the voltage range of 0.5-4.0V are presented in Figure 7b.In the initial cycle, NVP//MnFe 2 O 4 @4C SIB demonstrates a specific capacity of 111 mAh•g −1 at 1C.As illustrated in Figure 7c, the full cell exhibits favorable electrochemical performance, with a stable and reversible capacity of 78.5 mAh•g −1 over 35 cycles, and maintains a Coulombic efficiency of over 97% throughout the cycling process.

Synthesis of the MnFe2O4 Spherical Nanoparticles
The MnFe2O4 spherical nanoparticles were synthesized using a hydrothermal method based on the methods reported in [47], with some modifications.Initially, MnCl2•4H2O (0.396 g, 2 mmol) and FeCl3•6H2O (1.081 g, 4 mmol) were added to 80 mL of ethylene glycol and stirred to form a solution.Subsequently, NaAc (7.2 g) and polyethylene glycol (2.0 g, M = 20,000) were added.After stirring for 12 h, the resulting homogeneous solution was transferred into a 100 mL autoclave with a Teflon ® lining and heated at 200 °C for 10 h.After being cooled to room temperature, the resulting brown precipitate was collected via centrifugation at 10,000 rpm, washed several times with ultrapure water and absolute ethanol, and dried at 60 °C for 6 h.

Synthesis of the MnFe2O4@xC Spherical Nanocomposites
The carbon shell was coated onto the MnFe2O4 core to form MnFe2O4@xC core-shell nanocomposites by pyrolysis of PVDF [48,49].In general, 16.0 mg of PVDF was dissolved in a beaker containing 280.0 mg of N-methylpyrrolidone (NMP) with a PVDF mass ratio

Synthesis of the MnFe 2 O 4 Spherical Nanoparticles
The MnFe 2 O 4 spherical nanoparticles were synthesized using a hydrothermal method based on the methods reported in [47], with some modifications.Initially, MnCl 2 •4H 2 O (0.396 g, 2 mmol) and FeCl 3 •6H 2 O (1.081 g, 4 mmol) were added to 80 mL of ethylene glycol and stirred to form a solution.Subsequently, NaAc (7.2 g) and polyethylene glycol (2.0 g, M = 20,000) were added.After stirring for 12 h, the resulting homogeneous solution was transferred into a 100 mL autoclave with a Teflon ® lining and heated at 200 • C for 10 h.After being cooled to room temperature, the resulting brown precipitate was collected via centrifugation at 10,000 rpm, washed several times with ultrapure water and absolute ethanol, and dried at 60 • C for 6 h.

Table 1 .
Comparison of the SIB performance of the MnFe2O4@4C nanocomposite with other TMOs.

Figure 4 .
Figure 4. Rate capability (a) and cycling stability of MnFe 2 O 4 @1C, MnFe 2 O 4 O@4C, and MnFe 2 O 4 @8C at specific currents of (b) 0.1 A•g −1 and (c) 1.0 A•g −1 in a Na half-cell.2.3.Exploring the Origin of the Improvement in the Sodium-Ion Storage of the MnFe 2 O 4 @xC Nanocomposites Figure 5a illustrates the results of an EIS test on the three MnFe 2 O 4 @xC nanocomposites.The Nyquist curves of the three materials exhibit similar trends, comprising a semicircle in the high-frequency region and an inclined straight line in the low-frequency region (Figure5b).The cell resistances (R s , calculated from the intercept of the X-axis) for the MnFe 2 O 4 @1C, MnFe 2 O 4 @4C, and MnFe 2 O 4 @8C Na half cells are 1.86, 1.59, and 1.54 Ω, respectively.The diameters of the semicircles represent the charge transfer resistance (R ct ) coming from the electrode material/electrolyte interface reaction, which are 0.84, 0.70, and 0.62 Ω for the MnFe 2 O 4 @1C, MnFe 2 O 4 @4C, and MnFe 2 O 4 @8C nanocomposite electrodes, respectively.The R ct increases gradually with the increase in the thickness (content) of the carbon shell in the core-shell nanostructure due to the good electric conductivity of the carbon material, which improves the electric conductivity of the MnFe 2 O 4 @xC core-shell nanocomposites as a whole.Good electric conductivity leads to good rate capability.Moreover, as observed in Figures1 and 2, the particle connectivity of MnFe 2 O 4 @xC nanocompos-

Figure 5 .
Figure 5. (a) AC impedance measurements of MnFe 2 O 4 @1C, MnFe 2 O 4 @4C, and MnFe 2 O 4 @8C nanocomposite electrodes.The insert in (a) is the equivalent circuit diagram.(b) The zoomed−in area of the plots in the high-frequency region of (a).

Molecules 2024 , 13 Figure 6 .
Figure 6.(a) CVs at various scan rates.(b) Calculated b values.(c) Capacitive contribution at a scan rate of 1.0 mV•s −1 .(d) The ratio of capacitive contribution of the MnFe2O4@4C anode at various scan rates.

Figure 6 .
Figure 6.(a) CVs at various scan rates.(b) Calculated b values.(c) Capacitive contribution at a scan rate of 1.0 mV•s −1 .(d) The ratio of capacitive contribution of the MnFe 2 O 4 @4C anode at various scan rates.

Table 1 .
Comparison of the SIB performance of the MnFe 2 O 4 @4C nanocomposite with other TMOs.